Organic light emitting devices and organic transistor devices have been proposed utilizing various characteristics of organic materials such as light emitting and charge transport characteristics. By using organic materials for these devices, advantages such as light weight, low price, low manufacturing cost and flexibility are expected.
For materials for use in organic light emitting devices, various materials from low molecular weight materials to polymeric materials have been reported. As for low molecular weight materials, it is reported that efficiency is boosted by adopting various kinds of layer structures and durability is improved by skillfully controlling the doping method. However, in the case of layers of the low molecular weight materials, it is reported that the state of the layers changes with an extended period of time. Therefore the layers have a vital problem with stability of the film.
As for polymeric materials, pi-conjugated polymers such as poly-p-phenylenevinylene (PPV) based compounds and poly-thiophenes have been intensively studied. However, these materials have shortcomings such as difficulty in purifying the materials and a low fluorescence quantum yield. Currently, no high-performance light emitting device has been obtained so far.
Published unexamined Japanese Patent Applications Nos. 10-310635 and 8-157575, WO (Patent Cooperation Treaty) 97/09394 and Synth. Met., 84, 269, 1997 have disclosed polymeric materials which contain an aryl amine moiety in the main chain of pi-conjugated polymers. Considering that polymeric materials are inherently stable in the glass state, if a high fluorescent quantum yield is imparted thereto, an excellent light emitting device can be designed. Therefore, further improvement in this area has been sought.
As for organic thin film transistor (TFT) devices, various kinds of materials from low molecular weight materials to polymeric materials have been reported. Recently organic TFTs having an organic active layer have drawn much attention as a low cost replacement for silicon-based TFTS. By utilizing organic materials, thin films and circuits can be easily formed by wet methods such as printing methods, spin coating methods and dipping methods. The devices can thus be manufactured without the costly processes which are required in the manufacturing process of silicon-based TFTs. Therefore, large cost cutting in manufacturing and large-sized devices are expected. In addition, devices including organic materials have advantages such as mechanical flexibility and light weight. Although organic materials do not match with inorganic semiconductor materials in terms of carrier mobility, organic semiconductor devices attract intense interest due to the advantages mentioned above.
Next, structures and operations of organic TFTs will be described.
FIG. 1A illustrates a cross section of an example of conventional TFTs. The materials and the structure thereof will be explained referring to FIG. 1A.
In FIG. 1, numeral references 1, 2, 3, 4, 5 and 6 denote a source electrode (hereinafter a source), a drain electrode (hereinafter referred to as a drain), a gate electrode (hereinafter referred to as a gate), an organic semiconductor layer, an insulation layer and a substrate, respectively. When a voltage is applied between the source 1 and the drain 2, an electric current flows between the source 1 and the drain 2 through an organic semiconductor layer. At the same time, if a voltage is applied to the gate 3 which is separated from the organic semiconductor layer 4 by the insulation layer 5 therebetween, conductivity of the organic semiconductor layer varies because of the electric field effect, meaning that the electric current flowing between the source 1 and the drain 2 can be modulated. This is thought to be because the voltage which is applied to the gate 3 changes the width of the accumulation layer in the organic semiconductor layer adjacent to the insulation layer, resulting in change of the channel cross sectional area.
Specific examples of such proposed organic TFT materials include low molecular weight materials such as pentacene (Synth. Met., 51, 419, 1992.), phthalocyanine (Appl. Phys. Lett., 69, 3066, 1996.), fullerene (Published unexamined Japanese Patent Application No.8-228034 and Appl. Phys. Lett., 67, 121, 1995.), anthradithiophene (Published unexamined Japanese Patent Application No.11-195790), thiophene oligomer (Japanese Patent No.3145294 and Chem. Mater., 10, 457, 1998) and bis-dithienothiophene (Appl. Phys. Lett., 71, 3871, 1997.), and polymeric materials such as polythiophene (Appl. Phys. Lett., 69, 4108, 1996) and polythienylenevinylene (Appl. Phys. Lett., 63, 1372, 1993.).
The materials mentioned above have sufficient carrier mobility as to be used organic semiconductor for TFT devices. However, these materials still need improvements I order that the TFT devices using the organic materials satisfy commercial purposes. For example, it is reported that pentacene has a mobility of about 1 cm2/Vs. However, pentacene is barely soluble in a solvent and thereby forming a pentacene film from a solution is difficult. In addition, pentacene is unstable under oxidation conditions and therefore tends to be oxidized with time in an atmosphere containing oxygen. Similarly, materials such as phthalocyanine and fullerene are also barely soluble and generally an organic semiconductor layer thereof needs to be manufactured by a vacuum deposition method. Therefore, advantages of the devices using organic materials such as cost cutting in manufacturing process and formability of large-sized devices cannot be expected. Further, these materials have problems in that a film thereof tends to peel off and crack due to deformation of the substrate.
Among materials which can be applied by a wet process and have a relatively high mobility, polyalkylthiophene (Appl. Phys. Lett., 69, 4108, 1996.) based materials attract interest but have shortcomings such that a device using a polyalkylthiophene based compound has a low on-off ratio and polyalkylthiophene based compounds tend to be oxidized, and thereby the properties thereof change with time.
As described above, a plurality of materials have been proposed as organic semiconductor materials for TFTs but desirable organic semiconductor materials which satisfy all the requirements have not yet been obtained.
Therefore, a need exists for an organic semiconductor material which has excellent transistor characteristics, a solubility to a degree such that an excellent film can be formed in a wet process, and a stable preservability inclusive of oxidation resistance.